Hydrogen is a promising energy source due to the high calorific value supplied, besides being a product causing barely any pollution of the environment.
Hydrogen or hydrogen-rich gases, called synthesis gases, are produced on a large scale for use in the refining industry in processes for hydrofining of streams derived from petroleum, such as gasoline or diesel, ensuring that their quality meets the current standards of environmental legislation. Hydrogen also finds extensive application in the petrochemical industry for production of synthetic fuels (GTL), methanol, ammonia, urea and other widely used products.
There are various industrial processes for converting natural gas and other hydrocarbons to synthesis gas, but steam reforming is the main process for producing hydrogen on an industrial scale. The main reactions that occur in the steam reforming process are presented below:CnHm+nH2O═nCO+(n+½mnH2 (endothermic reaction)  Reaction 1CH4+H2O═CO+3H2 (endothermic, 206.4 kJ/mol)  Reaction 2CO+H2O═CO2+H2 (exothermic, −41.2 kJ/mol)  Reaction 3
The steam reforming process is usually carried out by introducing the hydrocarbon feed, selected from natural gas, refinery gas, liquefied petroleum gas, propane, butane or naphtha, purified beforehand by removing sulphur compounds, chlorides, heavy metals and/or olefins, and steam in excess relative to stoichiometry (reactions 1, 2 and 3) in a variable number of reactors consisting of metal tubes with typical dimensions of outside diameter from 7 to 15 cm and height between 6 10 to 13 m containing catalysts. These tubes are located inside a furnace, which supplies the heat required for the reactions. The assembly consisting of reactors and heating furnace is called a primary reformer.
However, the main problem in the generation of hydrogen or synthesis gas relates to the catalyst employed. Among other requirements, the catalyst must be efficient, must possess reasonable stability over a long period of time, and must be resistant to carbon deposits and to temperature.
Therefore the appropriate choice of catalyst has direct consequences for the costs of the process for producing hydrogen or synthesis gas. Accordingly, the use of more efficient catalytic systems and/or optimization of their performance in traditional processes are beginning to be of fundamental importance.
The catalysts used industrially in the steam reforming process typically consist of species of nickel oxide deposited on a refractory support. The supports used commercially are alpha-alumina and refractory cements of the calcium aluminate and/or magnesium aluminate type, or a mixture thereof. As examples from the literature, we may cite documents US 2002/0329645 and U.S. Pat. No. 3,759,678, which teach that nickel-based steam reforming catalysts are often prepared with the metallic nickel phase supported on alumina. When used as a support, alumina, especially the crystalline form called alpha-alumina, is used for providing a large surface area where the higher-cost active phase based on nickel oxides is dispersed, which will then be converted to metallic nickel prior to industrial use.
However, the prior art teaches that it is desirable for the alumina to be as inert as possible when used as a support, since the migration of aluminium species present in the support during the steps of catalyst preparation cause a major difficulty in reduction of the nickel oxide species present in the support (Lundegaard, L. F. L. et al., Catalysis Today, 2015).
Furthermore, the steam reforming process employs a typical inlet temperature of the mixture of hydrocarbons and steam in the reactors of the primary reformer from 400 to 550° C., and an outlet temperature from 750 to 950° C., at typical pressures from 10 to 40 kgf/cm2. These harsh conditions lead to the use of expensive nickel alloys for making the tubes, which accounts for a large fraction of the fixed costs of the process. The tubes are designed to operate for 100 000 hours in the design conditions of temperature and pressure. However, the useful life of the tubes is greatly reduced if the tube wall temperature exceeds the design value (Catalytic Steam Reforming, J. R. Rostrup-Nielsen; Springer-Verlag, 1984). Besides reducing the useful life of the tubes, a high tube wall temperature may cause a reduction in the capacity of the unit and/or stoppage thereof for replacing the catalyst, with the aim of minimizing the risks of tube breakage in operation (AlChe Safety 2004, Common problems on primary reformers).
Therefore, despite being widely used in industry, nickel-based catalysts for producing hydrogen or synthesis gas undergo deactivation by various mechanisms, including coke formation, poisoning due to the presence of sulphur compounds and chloride in the feed, and thermal deactivation, also called sintering of the active phase, which arises from the natural tendency of metals to agglomerate when exposed to high temperatures. This reduces the efficiency of the catalysts, leading to an increase in the costs of production of hydrogen or synthesis gas or, in more severe cases of deactivation, compromising the process or even leading to faults in the metallurgy of the tubes (reactors), with significant costs resulting from stopping the unit and with risks from the standpoint of safety of the equipment and of personnel.
Thus, it is highly desirable for steam reforming catalysts to be developed with high activity and possessing properties that allow this activity to be maintained for long periods, typically more than 3 years, preferably 5 years, which are typical times for scheduled stops for maintenance of oil refinery units. A technical solution taught in the literature for reducing deactivation of the catalyst by the process designated as sintering (Applied Catalysis A: General 498 (2015) 117-125), which occurs through agglomeration of the active phase or of the support when exposed to high temperatures, relates to the use of noble metals as promoters, selected from Ir, Rh and Ru to form a metal alloy with nickel. This technical solution is also adopted for processes of partial oxidation combined with steam reforming, as taught in patents U.S. Pat. No. 8,916,492 B2 and US 2012/0329645 A1.
Following this same line, in “Improving the sintering resistance of Ni/Al2O3 steam reforming catalysts by promotion with noble metals”, 18th Brazilian Congress on Catalysis”, Fernando Morales Cano et al. teach the use of Rh, Ir and Ru as a promoter of a steam reforming catalyst of the Ni/α-alumina type for mitigating the sintering of the Ni active phase, the effect observed having been explained by the formation of metal alloys.
Document US 2002/0329645 A1 already teaches that the method of forming metal alloys between Ni and noble metals to reduce sintering is also applicable to the production of hydrogen in processes for steam reforming of hydrocarbons in the presence of oxygen, just like the partial oxidation process or the autothermal reforming process.
However, the use of noble metals in catalysts causes high costs, reducing their use in processes for large-scale production of hydrogen, such as those with an output above 10 000 Nm3/day. This solution is thus of reduced industrial applicability owing to the limited availability of the promoters based on noble metals and the high cost of the resultant final product.
Thus, it is noted that there is still a need to provide a process for preparing a catalyst that reduces the deactivation of the metallic nickel phase when exposed to high temperatures and presence of steam for use in a steam reforming process for producing hydrogen or synthesis gas.
As will be explained in more detail below, the present invention provides a practical and efficient solution to the problems of the prior art described above.